The present invention relates to a process for the preparation of phosphinic acid esters and to the use of the products prepared by this process.
Phosphinic acid esters are valuable synthetic building blocks and can be used, for example, for the preparation of polymers and plastics in order to obtain flame-resistant materials.
Thus, DE 26 52 007 A1 describes flame-resistant epoxy resins through incorporation of carboxyl-functional phosphinic acids. U.S. Pat. No. 5,399,428 A1 describes flame-resistant linear polyesters through incorporation of carboxyl-functional phosphinic acids.
DE 25 40 283 A1 describes the addition of phosphines onto .alpha.,.beta.-unsaturated carboxylic acids in the presence of aqueous hydrochloric acid, followed by oxidation.
DE 28 49 003 describes the preparation of phosphorus-containing cyanohydrin derivatives by the addition of phosphonous acid esters onto acroleincyanohydrin derivatives.
Phosphinic acid esters are obtained by adding phosphonous acid monoesters onto 1-olefins in the presence of peroxidic catalysts. However, the yields are only low. The addition of phosphonous acid monoesters onto activated double bonds in the presence of alkoxides as catalyst proceeds better. Suitable unsaturated compounds are .alpha.,.beta.-unsaturated carboxylic acid esters or nitriles, .alpha.,.beta.-unsaturated ketones and alkyl vinyl sulfones and vinyl acetate (Houben-Weyl, Volume 12/1, pp. 258-259).
The phosphonous acid monoesters themselves are prepared from phosphonous acid dihalides by reaction with alcohols or by hydrolysis and subsequent esterification.
Functional phosphinic acids are obtained by reacting phosphonous acid dihalides (dihalophosphines) with activated olefinic compounds, such as, for example, acrylic or methacrylic acid derivatives, followed by hydrolysis (Houben-Weyl, Volume 12/1, p. 230; K. K. Khairullin, T. I. Sobchuk, A. N. Pudovik, Zh. Obshch. Khim. 37, 710 (1967)). Byproducts formed in the hydrolysis with organic acids are their halides.
In addition, phosphonous acid dihalides can be reacted with alkyl halides in the presence of aluminum chloride (Houben-Weyl, Volume 12/1, p. 232).
Phosphinic acid esters can also be prepared from dialkyl phosphonites by the Michaelis-Arbuzov reaction. The abovementioned dialkyl phosphonites are in turn prepared from phosphonous acid dihalides and hydroxyl compounds.
The abovementioned phosphonous acid dihalides, which can be employed as starting materials for other syntheses, for example methyldichlorophosphine, have hitherto themselves been prepared in a complex synthesis from phosphorus trihalides and alkyl halides in the presence of aluminum chloride (Houben-Weyl, Volume 12/1, p. 306). The reaction is highly exothermic and can only be controlled with difficulty in industry. In addition, various byproducts are formed which, like some of the abovementioned starting materials, are toxic and/or corrosive, i.e. are highly undesired.